Type II Superconductivity

D. Saint-James, G. Sarma and E.J. Thomas, Type II Superconductivity, Pergamon, Oxford, p. 4 (1969). 

Key words Theories and Models of the Superconducting State Type II Superconductivity High-Tc Compounds... 

Keywords Theories and models of the superconducting state Type II superconductivity ... 

Figure 5 Magnetic field B outside and inside a Type II superconducting slab located in an applied magnetic field Sapp oriented parallel to its surface (from Ref. 8, p. 135)...

Figure 7 Temperature dependence of the relative values of the critical current density Jq T), critical field BdT), energy gap Eg, density of superconducting electrons ng, and penetration depth X (T) in the Type II superconducting state below the transition temperature Tc...

Saint-James, D., Thomas, E.J., Sarma, G, 1969. Type II Superconductivity. Pergamon Press, Oxford. 

Discovery of the 90+ K Superconductor "Paul" Chu and coworkers at the University of Houston (during October 1986) carried out the synthesis of (La1.xBax)CuOs.y (Type I) and (La1.xBax)2 Cu04.y (Type II) compounds and isolated superconducting phases exhibiting a sharp decrease in resistivity at 32 K. The best materials, however, showed only a 2% Meissner fraction. By applying pressure to one such product, their forte in superconductor research, they observed an increase in transition temperature of 8 degrees at 14 kbar pressure (see Figure 29). Chu, et al., submitted (156) these results to Physical Review Letters on 15 December 1986, and the publication appeared in the January 26, 1987 issue. 

In addition to a critical temperature and critical field, all superconductors have a critical current density, Jc, above which they will no longer superconduct. This limitation has important consequences. A logical application of superconductors is as current-carrying media. However, there is a limit, often a low one, to how much current they can carry before losing their superconducting capabilities. The relationship between Jc, He, and Te for a Type II superconductor is shown in Figure 6.32. Notice that the Hc-Tc portion of this plot has already been presented in Figure 6.10 for a Type I superconductor. 

Here we will summarize, from the previous subsections as well as from literature, some typical properties and representative parameters (see table 6) of the superconducting state of YNi2B2C and LuNi2B2C where completeness is not attempted. These materials are usually clean-limit type II superconductors. However by substitutional disorder on the rare earth site in (Y,Lu)Ni2B2C or on the transition-metal site in Lu(Ni,Co)2B2C the residual resistance ratio RRR = p(300 K)/p(Tc), where p(T) is the normal state resistivity, and the mean free path / of the electrons in the normal state can be considerably reduced... 

Conductor-Superconductor Transition When some metals or compounds are cooled below a certain temperature, their electrical resistance drops abruptly to zero. This temperature is referred to as the superconducting transition temperature. These materials are classified into two categories, type I or type II superconductors, depending upon how a bulk sample behaves in an external magnetic field. In the absence of an external magnetic field, the (superconductor + normal) transition is continuous in both types of superconductors. When a magnetic field is applied, the transition becomes first order in type I superconductors, but remains continuous in the type II superconductors. 

The behavior of type II superconductors in an applied field is more complicated. This behavior is illustrated in Figure 13.16b. Below a certain field strength 2 c, i, the magnetic field lines are repelled, and the material is superconducting. From 2 c, i to a second critical field strength magnetic field lines are able to penetrate the type II material in its superconducting state... 

Figure 13.16 Magnetization verses applied magnetic field for (a) a type I superconductor and (b) a type II superconductor. For the type I superconductor, the magnetic flux does not penetrate the sample below 9 Cc where the sample is a superconductor. Above rMc, the sample is a normal conductor. For the type II superconductor, the magnetic field starts to penetrate the sample at 3Cc, 1, a magnetic field less than rXc, the thermodynamic critical field. Superconductivity remains in the so-called vortex state between 9 c and Ci2 until WCt2 is attained. At this magnetic field, complete penetration occurs, and the sample becomes a normal conductor.

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